This invention concerns aqueous solutions for forming crystalline phosphate coatings on metal surfaces. Conventional techniques for forming crystalline phosphate coatings are described in the following publications and references cited therein, the contents of which are incorporated herein by reference:
METALS HANDBOOK, 8th Ed. Vol. 2 ASM (1964) Pgs. 531-547;
ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, 2nd Ed. V. 13 John Wiley & Sons, Inc. (1967) Pgs. 293-297;
SURFACE PREPARATION AND FINISHES FOR METALS McGraw-Hill Book Co. (1971) Pgs. 396-401.
These techniques are generally based upon the formation of crystalline coatings containing insoluble zinc or manganese phosphate. The dihydrogen phosphates of these metals are soluble under bath conditions. During processing, however, the monohydrogen phosphate salt is formed which contributes to sludge formation to a substantial extent.
One of the crucial problems relative to the process of crystalline phosphatization lies in the accumulation of this insoluble sludge which settles either on the bottom of the tank containing the phosphatizing solution or on the heating elements. The accumulation on the heating elements lowers the heat exchange efficiency and creates problems in regard to maintaining the temperature of the bath. The settling of sludge on the bottom of the tank reduces the necessary amount of clear phosphatizing liquid making it difficult to obtain the desired coatings and results in consumption of active chemicals. Some sludge may settle on the metal surface causing the formation of a nonadherent "dusting", which lowers the quality of the surface coating.
If left unchecked, the sludge buildup will eventually necessitate dumping the treating solution and cleaning the tank. This procedure is costly and time consuming, and is therefore preferably performed as infrequently as possible. Physical means such as filtration or decanting have been employed to lengthen the useful life of baths but these techniques create new problems and do not avoid all of the above-mentioned sludge problems.
The purpose of this invention is to reduce the disadvantages of sludge formation by providing a way of reducing the rate of formation of sludge during the phosphatization treatment. U.S. Pat. Nos. 2,304,299; 2,744,555 and 2,804,498 refer to the inclusion of phosphate esters in phosphatizing baths but do not suggest that the particular esters employed in the present invention will reduce the rate of sludge content in a crystalline phosphatizing bath.